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Implementing Dynamic Finders and Parsing Method Expressions

Most ORMs support the concept of dynamic finders. A dynamic finder looks like a normal method invocation, but the method itself doesn’t exist, instead, it’s generated dynamically and processed via another method at runtime.

A good example of this is Ruby. When you invoke a method that doesn’t exist, it raises a NoMethodError exception, unless you define “method_missing”. Rails ActiveRecord::Base class implements some of its magic thanks to this method. For example, find_by_title(title) and find_by_title_and_date(title, date) are turned into:

What’s nice about Ruby is that the language allows you to define methods dynamically using the “define_method” method. That’s how Rails defines each dynamic finder in the class after it is first invoked, so that future attempts to use it do not run through the “method_missing” method.

Method Expressions

GORM, Grails ORM library, introduces the concept of dynamic method expressions. A method expression is made up of the prefix such as “findBy” followed by an expression that combines one or more properties. Grails takes advantage of Groovy features to provide dynamic methods:

findByTitle("Example")
findByTitleLike("Exa%")

Method expressions can also use a boolean operator to combine two criteria:

findAllByTitleLikeAndDateGreaterThan("Exampl%", '2010-03-23')

In this case we are using AND in the middle of the query to make sure both conditions are satisfied, but you could equally use OR:

findAllByTitleLikeOrDateGreaterThan("Exampl%", '2010-03-23')

Parsing Method Expressions

MethodExpressionParser is a PHP library for parsing method expressions. It’s designed to quickly and easily parse method expressions and construct conditions based on attribute names and arguments.

findByTitleAndDate('Example', date('Y-m-d'));
SELECT * FROM book WHERE title = ? AND date = ?
findByTitleOrDate('Example', date('Y-m-d'))
SELECT * FROM book WHERE title = ? OR date = ?
findByPublisherOrTitleAndDate('Name', 'Example', date('Y-m-d'))
SELECT * FROM book WHERE publisher = ? OR (title = ? AND date = ?)
findByPublisherInAndTitle(array('Name1', 'Name2'), 'Example')
SELECT * FROM book WHERE publisher IN (?, ?) AND date = ?
findByTitleLikeAndDateNotNull('Examp%')
SELECT * FROM book WHERE title LIKE ? AND date NOT NULL
findByIdOrTitleAndDateNotNull(1, 'Example')
SELECT * FROM book WHERE (id = ?) OR (title = ? AND date NOT NULL)

Performance

PHP doesn’t allow you to define methods dynamically, this means that every time you invoke a finder method the parser has to search, extract and map all the attribute names and expressions. To avoid introducing this performance overhead you can cache the attribute names. For example: